Human joints can become damaged as a result of accident or illness. Such damage can be, for example, to the articular cartilage covering the ends of the bones at the joint as well as the intra-articular cartilage between the ends of the adjacent bones of the joint. When the damage to the joint is severe, a joint endoprosthesis can be implanted to improve the comfort and mobility of the patient.
Joint endoprostheses have been developed to replace native tissue of several human joints. There are a variety of knee prostheses, hip prostheses, shoulder prostheses, ankle prostheses, elbow prostheses, wrist prostheses and finger prostheses available to relieve patient suffering. Such devices are available, for example, from DePuy Orthopaedics, Inc. (DePuy Products, Inc.) of Warsaw, Ind.
Standard joint endoprostheses include components that are affixed to the articulating ends of the bones of the joint and commonly include a bearing component positioned in the joint space between the bone-affixed components. Standard bearing components of joint endoprostheses have a surface against which one of the bone-affixed components articulates. For example, hip endoprostheses include a proximal femoral component to be affixed to the proximal femur and an acetabular cup or shell to be affixed to the acetabulum. Many of these standard hip endoprostheses include a liner in the acetabular cup against which the head of the proximal femoral component articulates. Knee prostheses commonly include a distal femoral component to be affixed to the distal femur and a proximal tibial component to be affixed to the proximal tibia. Tibial bearings are typically in the joint space between the femoral and tibial components, carried by the tibial component. Similar systems with bearings are available to replace other joints in the body.
Standard bearings for joint endoprostheses are made of ultrahigh molecular weight polyethylene (UHMWPE), ceramic and metal. Bearing wear is problematic in the orthopaedic field. Several patents have addressed the problem particles produced by UHMWPE wear, and the association of these wear particles with osteolysis. See, for example: U.S. Pat. No. 6,281,264, “Chemically Crosslinked Ultrahigh Molecular Weight Polyethylene for Artificial Human Joints” and U.S. Pat. No. 6,228,900 “Crosslinking of Polyethylene for Low Wear Using Radiation and Thermal Treatments.” In addition, the bearing can wear in an uneven manner if the soft tissue of the joint is or becomes unbalanced. For example, if the soft tissue of the knee is unbalanced, the medial or lateral side of the tibial bearing could be subjected to additional stress and wear.
Osteolysis at the site of the orthopaedic implant is associated with loosening of the components of the joint endoprosthesis. Such loosening of the components can require a revision surgery, wherein the original joint endoprosthesis components are removed and replaced.
Orthopaedic surgeons have monitored the condition of the joint space occupied by bearings to monitor the condition of the patient. Typically, narrowing of the joint space has been monitored from measurements taken from radiographs or by use of radio stereophotogrammetric analysis (RSA). Both methods require use of external imaging devices, and both can be time consuming. Both methods are prone to some measurement error due to the limited accuracy of the equipment. Also, the accuracy of measurements of joint space taken from radiographs depends on analysis technique and bias, affecting repeatability and reproducibility. In addition, because of the high equipment cost, it is unlikely that RSA will become a standard technique in patient monitoring.
Early detection of bearing wear could find use as a signal to the orthopaedic surgeon that some type of intervention is needed before the condition degenerates to the point of requiring revision surgery. For example, the surgeon may determine that the patient needs to make some lifestyle changes if revision is to be postponed. Moreover, if therapeutic agents to treat early stage osteolysis are available, these agents could be administered before the condition degenerates to the point where revision surgery is necessary.
Reference is made to the following United States patents, the disclosures of which are incorporated by reference herein in their entireties: U.S. Pat. No. 6,583,630 to Mendes et al. entitled “Systems and Methods for Monitoring Wear and/or Displacement of Artificial Joint Members, Vertebrae, Segments of Fractured Bones and Dental Implants”; U.S. Pat. No. 6,573,706 to Mendes et al. entitled “Method and Apparatus for Distance Based Detection of Wear and the Like in Joint”; and U.S. Pat. No. 6,245,109 to Mendes et al. entitled “Artificial Joint System and Method Utilizing Same for Monitoring Wear and Displacement of Artificial Joint Members.”